Spring motors are well-known and generally comprise a flat ribbon of spring metal which is pre-stressed on one side and coiled so as to have a natural or relaxed state in which the spring forms a tightly wound coil. The coil is normally disposed on or in a storage or take-up drum. The free end of the coil is attached to the hub of an output or drive drum onto which the spring is back wound by rotating the output drum in the direction to back wind the spring thereon. When the holding force by which the spring is back wound on the output drum is released, the curling property of the spring rewinds the latter onto or into the storage or take-up drum towards its natural or relaxed state. This resulting motion can be utilized in a number of applications requiring reciprocating motion such as with seat belts, cordless blinds and the like.
The spring member in such spring motors can be of constant or variable force depending upon the intended use for the motor. The variable force characteristic can be obtained in a number of ways including tapering the width and or thickness of the spring as disclosed in U.S. Pat. Nos. 5,482,100 and 5,531,257. Such patents are directed to the use of spring motors in conjunction with a cordless window blind.
In the aforementioned patents, a cord drum is provided lateral to, and concentric with, the drive drum such that rotation of the cord spool causes rotation of the drive drum. Rotation of the drive drum in turn uncoils the spring from the take-up drum and back winds the spring onto the drive drum. A cord is connected between the cord drum and the bottom rail of the blind. When the blind is extended downwardly, this rotation pulls the cord from the cord spool which in turn rotates the cord spool, the drive drum, and the take-up drum. The spring motor facilitates smooth motion of the bottom rail.
In still further devices, the cord spool or spools are provided in a linear arrangement with the axis of the take-up drum, drive drum and cord spools being parallel. Typically, two cord spools are provided, one adjacent the drive drum, and one adjacent the take-up drum. In order to transfer torque from the cord spools to the drive drum, the cord spools are provided with radially extending gear teeth which mesh with mating teeth on the drive drum. In one case, the cord spool teeth mesh with teeth of an idler gear mounted proximate the take-up drum, and the idler gear then meshes with the drive drum. The take-up drum and idler gear are mounted on the same axis and concentric with one another, with the take-up drum being provided with a recess on one side about which the idler gear rotates.
While such systems are workable, the idler gear and take-up drum can tend to bind and not freely rotate, which necessarily detrimentally affects performance of the coil spring. More specifically, if the take-up drum is not allowed to freely rotate, the coil can tend to "grow" in that it is not tightly wound about the take-up drum. As a result the coil spring loses force and may become disengaged from the take-up drum. Such binding can occur because the rotational force on the gear necessarily side loads the axle or hub or around which it rotates causing excess friction and/or disproportionate rotation of the idler spool.